Least Bell's Vireo

views updated

Least Bell's Vireo

Vireo bellii pusillus

ListedMay 2, 1986
FamilyVireonidae (Vireo)
DescriptionSmall songbird, gray above, white below.
HabitatWillow-dominated brush.
ReproductionClutch of three or four eggs.
ThreatsDestruction of riparian woodlands, cowbird brood parasitism.


Vireo bellii pusillus (least Bell's vireo), a subspecies of Bell's vireo, is a migratory songbird about 4.8 in (12 cm) long. It is gray above, white below and has inconspicuous white spectacles.

Three other subspecies of Bell's vireo are recognized by the American Ornithologists' Union : V. b. bellii of the midwestern U. S., V. b. medius of Texas, and V. b. arizonae of the southwestern U. S. and northern Mexico. While all are similar in appearance, behavior, and life history, subspecies' breeding ranges are geographically distinct.


Least Bell's vireo has a loud and persistent song. It builds cuplike nests 3 ft (1 m) off the ground between forking twigs, usually in dense brush along willow-dominated streambanks. It lays three or four spotted eggs that hatch in about 14 days. The young remain in the nest approximately 10-12 days. The diet consists of insects. Virtually all Bell's vireos winter in Mexico, arriving at the breeding ground mid-March to early April and departing in late August or September.


The least Bell's vireo occupies a more restricted nesting habitat than other subspecies. It prefers dense, willow-dominated areas adjacent to streams and having lush understory vegetation. The range of other Bell's vireo subspecies extends into upland desert scrub

The least Bell's vireo nests primarily in willows, but it will use other trees and shrubs. It forages along streambeds and in adjoining chaparral (scrub oak) habitat, usually staying within 900 ft (275 m) of the nest.


Once widespread and abundant throughout California's Central Valley and other low-elevation riverine valleys, this vireo's historical breeding range extended from Red Bluff (Tehama County) in interior northern California to northwestern Baja California, Mexico.

Numerous detections of this Endangered bird throughout southern California in the spring and early summer of 1994 were the first indications that the bird is at last expanding its range and may be on the road to recovery. In the Prado Basin (River-side County), at least 150 vireo pairs were detected thus by mid-1994 in an area where 19 pairs were detected in 1986. By the end of 1994, more than 1,000 males had been heard singing along rural southern California rivers and streams. Preliminary data suggested that at least two large populations elsewhere have similarly increased in size. Vireos that were color-marked by managers in San Diego County continue to appear and breed in areas 80 mi (130 km) or more to the north in Riverside and Orange Counties.


No other passerine (perching songbird) species in California is known to have declined as dramatically as the least Bell's vireo, although by the mid-1990s, the species was showing clear signs of recovery. In 1986 a management plan was created to protect at least 20,000 acres (8,065 hectares) of least Bell's vireo habitat in 12 California locations. A 1988 census indicated that most of the larger vireo populations had increased significantly over the previous year. Smaller populations (those numbering less than ten) were hanging on precariously. Widespread loss of riparian habitats and brood parasitism by the brown-headed cowbird (Molothrus ater ) were the main causes of the vireo's decline. By 1990, the bird had been extirpated from an estimated 95% of its former range largely as a result of cowbird parasitism. But by 1994, population numbers were growing fast and recovery efforts seemed to be paying off.

Conservation and Recovery

Three vireo nesting areas along the San Luis Rey River are being maintained by the California Department of Transportation to compensate for habitat damage caused during the widening of a State highway, and by the Army Corps of Engineers to compensate for damage from a flood control project. In those three sites, 22 vireo males established breeding territories. Along the entire San Luis Rey River, from Interstate 15 to the Pacific Ocean, the number of territorial vireo males had grown from about 40 in the mid-1980s to 142 in 1994.

Similar population growth has been documented in vireo habitats elsewhere, and birds from the largest vireo populations are re-colonizing historic habitats. Vireos that were color-marked in managed San Diego County areas are appearing and attempting to breed in areas over 80 mi (130 km) to the north in Riverside and Orange Counties.

Cowbird management has been critical to this recovery because cowbirds practice brood parasitismlaying their eggs in the nests of smaller birds like the vireo. Cowbird eggs hatch first, and their larger chicks eat most of the food vireo parents bring to the nest. Cowbird chicks may also crowd vireo eggs and chicks out of the nest. Many songbirds in the eastern and midwestern United States have evolved defenses against this kind of parasitism. Cowbirds have been in California for only about 75 years, however, so vireos have not evolved a defense.

Hundreds to thousands of cowbirds and their eggs are removed each year. Once the vireo's population becomes large and healthy enough to sustain the cowbird's onslaught, the control efforts can stop.

Biologists are encountering new problems, however, that illustrate the challenges to habitat restoration and vireo recovery. Homeless people are now living in wire-mesh cowbird traps or dismantling them in hopes of selling the parts. Encampments of people along the San Diego, San Luis Rey, and other rivers in San Diego County are also destroying breeding habitat, disturbing nesting birds, and trampling nests.

Despite aggressive efforts to protect the habitat, unauthorized activities in wetlands continued into the 1990s; in the Prado Basin in September, 1990, for instance, the installation of a pipeline in willow woodland along Chino Creek in violation of the terms of the utility's permit, destroyed at least 2-3 acres (0.8-1.2 hectares) of wetland vegetation and at least one nesting locale used by a pair of vireos during the 1990 breeding season. This was the sixth incident in five years involving the destruction of occupied least Bell's vireo habitat within the Prado Basin. Following that episode, the U.S Fish and Wildlife Service stepped up its work with the Army Corps of Engineers' Regulatory Branch to stop unauthorized wetlands activities.

The ultimate recovery goal is to have the vireo firmly reestablished in at least one-third of its former range in California before it can be considered for removal from the endangered species list. It appears the vireo is well on the way to reaching that goal.


U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Building
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121


Garrett, K., and J. Dunn. 1981. Birds of Southern California: Status and Distribution. Los Angeles Audubon Society, Los Angeles.

Goldwasser, S. 1978. "Distribution, Reproductive Success and Impact of Nest Parasitism by Brown-Headed Cowbirds on Least Bell's Vireos." Pamphlet of the California Department of Fish and Game, Sacramento.

Goldwasser, S., D. Gaines, and S. Wilbur. 1980. "The Least Bell's Vireo in California: A De Facto Endangered Race." American Birds 34:742-745.

Wilbur, S. 1980. "The Least Bell's Vireo in Baja California, Mexico." Western Birds 11:129-133.